Wound care is a major clinical challenge. Healing and chronic non-healing wounds are associated with a number of biological tissue changes including inflammation, proliferation, remodeling of connective tissues and, a common major concern, bacterial infection. A proportion of wound infections are not clinically apparent and contribute to the growing economic burden associated with wound care, especially in aging populations. Currently, the gold-standard wound assessment includes direct visual inspection of the wound site under white light combined with indiscriminate collection of bacterial swabs and tissue biopsies resulting in delayed, costly and often insensitive bacteriological results. This may affect the timing and effectiveness of treatment. Qualitative and subjective visual assessment only provides a gross view of the wound site, but does not provide information about underlying biological and molecular changes that are occurring at the tissue and cellular level. A relatively simple and complementary method that exploits ‘biological and molecular’ information to improve the early identification of such occult change is desirable in clinical wound management. Early recognition of high-risk wounds may guide therapeutic intervention and provide response monitoring over time, thus greatly reducing both morbidity and mortality due especially to chronic wounds.
Wound care and management is major clinical challenge that presents a significant burden and challenge to health care globally [Bowler et al., Clin Microbiol Rev. 2001, 14:244-269; Cutting et al., Journal of Wound Care. 1994, 3:198-201; Dow et al., Ostomy/Wound Management. 1999, 45:23-40]. Wounds are generally classified as, wounds without tissue loss (e.g. in surgery), and wounds with tissue loss, such as burn wounds, wounds caused as a result of trauma, abrasions or as secondary events in chronic ailments (e.g., venous stasis, diabetic ulcers or pressure sores and iatrogenic wounds such as skin graft donor sites and dermabrasions, pilonidal sinuses, non-healing surgical wounds and chronic cavity wounds). Wounds are also classified by the layers involved, superficial wounds involve only the epidermis, partial thickness wounds involve only epidermis and dermis, and full thickness wounds involve the subcutaneous fat or deeper tissue. Although restoration of tissue continuity after injury is a natural phenomenon, infection, quality of healing, speed of healing, fluid loss and other complications that enhance the healing time represents a major clinical challenge. The majority of wounds heal without any complication. However, chronic non-healing wounds involving progressively more tissue loss result in a large challenge for wound-care practitioners and researchers. Unlike surgical incisions where there is relatively little tissue loss and wounds generally heal without significant complications, chronic wounds disrupt the normal process of healing which is often not sufficient in itself to effect repair. Delayed healing is generally a result of compromised wound physiology [Winter (1962) Nature. 193:293-294] and typically occurs with venous stasis and diabetic ulcers, or prolonged local pressure as in immuno-suppressed and immobilized elderly individuals. These chronic conditions increase the cost of care and reduce the patient's quality of life. As these groups are growing in number, the need for advanced wound care products will increase.
Conventional clinical assessment methods of acute and chronic wounds continue to be suboptimal. They are usually based on a complete patient history, qualitative and subjective clinical assessment with simple visual appraisal using ambient white light and the ‘naked eye’, and can sometimes involve the use of color photography to capture the general appearance of a wound under white light illumination [Perednia (1991) J Am Acad Dermatol. 25: 89-108]. Regular re-assessment of progress toward healing and appropriate modification of the intervention is also necessary. Wound assessment terminology is non-uniform, many questions surrounding wound assessment remain unanswered, agreement has yet to be reached on the key wound parameters to measure in clinical practice, and the accuracy and reliability of available wound assessment techniques vary. Visual assessment is frequently combined with swabbing and/or tissue biopsies for bacteriological culture for diagnosis. Bacterial swabs are collected at the time of wound examination and have the noted advantage of providing identification of specific bacterial/microbial species [Bowler, 2001; Cutting, 1994; Dow, 1999; Dow G. In: Krasner et al. eds. Chronic Wound Care: A Clinical Source Book for Healthcare Professionals, 3rd ed. Wayne Pa.: HMP Communications. 2001:343-356]. However, often, multiple swabs and/or biopsies are collected randomly from the wound site, and some swabbing techniques may in fact spread the microorganisms around with the wound during the collection process thus affecting patient healing time and morbidity [Dow, 1999]. This may be a problem especially with large chronic (non-healing) wounds where the detection yield for bacterial presence using current swabbing and biopsy protocols is suboptimal (diagnostically insensitive), despite many swabs being collected. Thus, current methods for obtaining swabs or tissue biopsies from the wound site for subsequent bacteriological culture are based on a non-targeted or ‘blind’ swabbing or punch biopsy approach, and have not been optimized to minimize trauma to the wound or to maximize the diagnostic yield of the bacteriology tests. In addition, obtaining swabs and biopsy samples for bacteriology can be laborious, invasive, painful, costly, and more importantly, bacteriological culture results often take about 2-3 days to come back from the laboratory and can be inconclusive [Serena et al. (2008) Int J Low Extrem Wounds. 7(1):32-5.; Gardner et al., (2007) WOUNDS. 19(2):31-38], thus delaying accurate diagnosis and treatment [Dow, 1999]. Thus, bacterial swabs do not provide real-time detection of infectious status of wounds. Although wound swabbing appears to be straightforward, it can lead to inappropriate treatment, patient morbidity and increased hospital stays if not performed correctly [Bowler, 2001; Cutting, 1994; Dow, 1999; Dow, 2001]. The lack of a non-invasive imaging method to objectively and rapidly evaluate wound repair at a biological level (which may be at greater detail than simply appearance or morphology based), and to aid in targeting of the collection of swab and tissue biopsy samples for bacteriology is a major obstacle in clinical wound assessment and treatment. An alternative method is highly desirable.
As wounds (chronic and acute) heal, a number of key biological changes occur at the wound site at the tissue and cellular level [Cutting, 1994]. Wound healing involves a complex and dynamic interaction of biological processes divided into four overlapping phases—haemostasis, inflammation, cellular proliferation, and maturation or remodeling of connective tissues—which affect the pathophysiology of wound healing [Physiological basis of wound healing, in Developments in wound care, PJB Publications Ltd., 5-17, 1994]. A common major complication arising during the wound healing process, which can range from days to months, is infection caused by bacteria and other microorganisms [Cutting, 1994; Dow, 1999]. This can result in a serious impediment to the healing process and lead to significant complications. All wounds contain bacteria at levels ranging from contamination, through colonization, critical colonization to infection, and diagnosis of bacterial infection is based on clinical symptoms and signs (e.g., visual and odorous cues).
The most commonly used terms for wound infection have included wound contamination, wound colonisation, wound infection and, more recently, critical colonisation. Wound contamination refers to the presence of bacteria within a wound without any host reaction [Ayton M. Nurs Times 1985, 81(46): suppl 16-19], wound colonisation refers to the presence of bacteria within the wound which do multiply or initiate a host reaction [Ayton, 1985], Critical colonisation refers to multiplication of bacteria causing a delay in wound healing, usually associated with an exacerbation of pain not previously reported but still with no overt host reaction [Falanga et al., J Invest Dermatol 1994, 102(1): 125-27; Kingsley A, Nurs Stand 2001, 15(30): 50-54, 56, 58].
Wound infection refers to the deposition and multiplication of bacteria in tissue with an associated host reaction [Ayton, 1985]. In practice the term ‘critical colonisation’ can be used to describe wounds that are considered to be moving from colonisation to local infection. The challenge within the clinical setting, however, is to ensure that this situation is quickly recognized with confidence and for the bacterial bioburden to be reduced as soon as possible, perhaps through the use of topical antimicrobials. Potential wound pathogens can be categorised into different groups, such as, bacteria, fungi, spores, protozoa and viruses depending on their structure and metabolic capabilities [Cooper et al., Wound Infection and Microbiology.: Medical Communications (UK) Ltd for Johnson & Johnson Medical, 2003]. Although viruses do not generally cause wound infections, bacteria can infect skin lesions formed during the course of certain viral diseases. Such infections can occur in several settings including in health-care settings (hospitals, clinics) and at home or chronic care facilities. The control of wound infections is increasingly complicated, yet treatment is not always guided by microbiological diagnosis. The diversity of micro-organisms and the high incidence of polymicrobic flora in most chronic and acute wounds gives credence to the value of identifying one or more bacterial pathogens from wound cultures. The early recognition of causative agents of wound infections can assist wound care practitioners in taking appropriate measures. Furthermore, faulty collagen formation arises from increased bacterial burden and results in over-vascularized friable loose granulation tissue that usually leads to wound breakdown [Sapico et al. (1986) Diagn Microbiol Infect Dis. 5:31-38].
Accurate and clinically relevant wound assessment is an important clinical tool, but this process currently remains a substantial challenge. Current visual assessment in clinical practice only provides a gross view of the wound site (e.g., presence of purulent material and crusting). Current best clinical practice fails to adequately use the critically important objective information about underlying key biological changes that are occurring at the tissue and cellular level (e.g., contamination, colonization, infection, matrix remodeling, inflammation, bacterial/microbial infection, and necrosis) since such indices are i) not easily available at the time of the wound examination and ii) they are not currently integrated into the conventional wound management process. Direct visual assessment of wound health status using white light relies on detection of color and topographical/textural changes in and around the wound, and thus may be incapable and unreliable in detecting subtle changes in tissue remodeling. More importantly, direct visual assessment of wounds often fails to detect the presence of bacterial infection, since bacteria are occult under white light illumination. Infection is diagnosed clinically with microbiological tests used to identify organisms and their antibiotic susceptibility. Although the physical indications of bacterial infection can be readily observed in most wounds using white light (e.g., purulent exudate, crusting, swelling, erythema), this is often significantly delayed and the patient is already at increased risk of morbidity (and other complications associated with infection) and mortality. Therefore, standard white light direct visualization fails to detect the early presence of the bacteria themselves or identify the types of bacteria within the wound.
Implantation and grafting of stem cells have recently become of interest, such as for wound care and treatment. However, it is currently challenging to track the proliferation of stem cells after implantation or grafting. Tracking and identifying cancer cells have also been challenging. It would be desirable if such cells could be monitored in a minimally-invasive or non-invasive way.
It is also useful to provide a way for detecting contamination of other target surfaces, including non-biological targets.